Any ophthalmic lens intended to be held in a frame involves a prescription. The ophthalmic prescription can include a positive or negative power prescription as well as an astigmatism prescription. These prescriptions correspond to corrections enabling the wearer of the lenses to correct defects of his vision. A lens is fitted in the frame in accordance with the prescription and with the position of the wearer's eyes relative to the frame.
In the simplest cases, the prescription is nothing more than a power prescription. The lens is said to be unifocal and has a rotational symmetry. It is fitted in a simple manner in the frame so that the principal viewing direction of the wearer coincides with the axis of symmetry of the lens.
For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition (or power progression) representing the power increment between far vision and near vision; this comes down to a far-vision power prescription and a near-vision power prescription. Lenses suitable for presbyopic wearers are progressive multifocal lenses; these lenses are described for example in FR-A-2 699 294, U.S. Pat. No. 5,270,745 or U.S. Pat. No. 5,272,495, FR-A-2 683 642, FR-A-2 699 294 or also FR-A-2 704 327. Progressive multifocal ophthalmic lenses include a far-vision zone, a near-vision zone and an intermediate-vision zone, a principal progression meridian crossing these three zones. They are generally determined by optimization, based on a certain number of constraints imposed on the different characteristics of the lens. These lenses are all-purpose lenses in that they are adapted to the different needs of the wearer at the time.
Families of progressive multifocal lenses are defined, each lens of a family being characterized by an addition which corresponds to the power variation between the far-vision zone and the near-vision zone. More precisely, the addition, referenced A, corresponds to the power variation between a point FV of the far-vision zone and a point NV of the near-vision zone, which are respectively called far-vision control point and near-vision control point, and which represent the points of intersection of viewing with the surface of the lens for far distance vision and for reading vision.
In one family of lenses the addition varies from one lens to the other in the family between a minimum addition value and a maximum addition value of 0.25 diopter and by 0.25 diopter from one lens to the other in the family.
Lenses with the same addition differ in the value of the mean sphere at a reference point, also called a base. It is possible to choose for example to measure the base at the point FV for measuring far vision. Thus the choice of a pair (addition, base) defines a group or set of aspherical front faces for progressive multifocal lenses. Generally, it is thus possible to define 5 base values and 12 addition values, i.e. sixty front faces. In each of the bases an optimization is carried out for a given power. Starting from semi-finished lenses, of which only the front face is formed, this known method makes it possible to prepare lenses suited to each wearer, by simple machining of a spherical or toric rear face.
Progressive multifocal lenses thus usually comprise an aspherical front face, which is the face away from the person wearing the glasses and a rear spherical or toric face directed towards the person wearing the glasses. This spherical or toric face allows the lens to be adapted to the user's ametropia, so that a progressive multifocal lens is generally defined only by its aspherical surface. As is well known, an aspherical surface is generally defined by the altitude of all of its points. The parameters constituted by the minimum and maximum curvatures at each point are also used, or more commonly their half-sum and their difference. This half-sum and this difference multiplied by a factor n−1, n being the refractive index of the lens material, are called mean sphere and cylinder.
A progressive multifocal lens can thus be defined, at every point on its complex surface, by geometric characteristics including a mean sphere value and a cylinder value, given by the following formulae.
In a manner known per se, at any point of a complex surface, a mean sphere D given by the formula:
  D  =                    n        -        1            2        ⁢          (                        1                      R            1                          +                  1                      R            2                              )      
is defined, where R1 and R2 are the maximum and minimum local radii of curvature expressed in meters, and n is the index of the material constituting the lens.
A cylinder C, given by the formula:
      C    =                  (                  n          -          1                )            ⁢                                            1                          R              1                                -                      1                          R              2                                                    ,
is thus defined.
The characteristics of the complex face of the lens can be expressed using the mean sphere and the cylinder.
Moreover, a progressive multifocal lens can also be defined by optical characteristics taking into account the situation of the wearer of the lenses. In fact, the laws of the optics of ray tracings mean that optical defects appear when the rays deviate from the central axis of any lens. Conventionally, the aberrations known as power defects and astigmatism defects are considered. These optical aberrations can be generically called obliquity defects of rays.
Obliquity defects of rays have already been clearly identified in the prior art and improvements have been proposed. For example, the document WO-A-98 12590 describes a method for determination by optimization of a set of progressive multifocal ophthalmic lenses. This document proposes defining the set of lenses in consideration of the optical characteristics of the lenses and in particular the wearer power and oblique astigmatism under wearing conditions. The lens is optimized by ray tracing, using an ergorama linking a target object point with each direction of viewing under wearing conditions.
EP-A-0 990 939 also proposes to determine a lens by optimization taking into account the optical characteristics instead of the surface characteristics of the lens. For this purpose the characteristics of an average wearer are considered, in particular as regards the position of the lens in front of the eye of the wearer in terms of curving contour, pantoscopic angle and lens-eye distance.
It is thus possible to consider, in addition to the obliquity defects of rays described previously, the so-called higher order optical aberration such as spherical aberrations or coma by studying the deformations which are undergone by a non-aberrant spherical wave front passing through the lens.
It is considered that the eye rotates behind the lens in order to sweep over all of its surface. Thus, at each point, an optical system composed of the eye and the lens is considered, as will be explained in detail below with reference to FIGS. 1 to 3. The optical system is therefore different at each point of the surface of the lens because the relative positions of the principle axis of the eye and of the lens are in fact different at each point due to the rotation of the eye behind the lens.
In each of these successive positions, the aberrations undergone by the wave front which passes through the lens and is limited by the pupil of the eye are calculated.
The spherical aberration results for example from the fact that the rays which pass at the edge of the pupil do not converge in the same plane as the rays which pass close to its centre. Moreover, the coma represents the fact that the image of a point situated outside the axis has a tail, due to the power variation of the optical system. Reference can be made to the article by R. G. Dorsch and P. Baumbach, “Coma and Design Characteristics of Progressive Addition Lenses” R. G. Dorsch, P. Baumbach, Vision Science and Its Applications, Santa Fe, February 1998 which describes the effects of coma on a progressive multifocal lens.